Edentulism, the condition of being toothless to some extent, may be treated by the implantation of a dental assembly. These assemblies require certain components to rest comfortably and securely in the patient's oral cavity. The implant fixture, also known as the dental implant or simply the implant, is the part of the dental assembly that becomes fused with the patient's jaw bone. The implant is available in both cylinder and screw-type varieties and is typically made from titanium or a titanium alloy. Implant abutments are screwed onto the implant and are positioned at and above the patient's gum line. Finally, a dental prosthesis is placed over the abutment and is designed to look and function like a natural tooth.
Thus, for the dental prosthesis to function as intended, the implant, which is the first component added to the patient's oral cavity, must be positioned with precision and in such a manner that it is securely bonded to the jaw bone until the process of osseointegration is complete. Screw-type implants (also called “implant screws”) enjoy widespread clinical use, in part, because they are capable of being securely fastened to the jaw bone. Implant screws are inserted into the jaw bone by linear motion through rotation after a uniform bore hole is pre-drilled into the bone.
Implant screws can be separated into one of four basic categories, as shown in FIGS. 1-4, based on the geometry of their threads: V-shaped, buttress, reverse buttress, and square-shaped. These various geometries are defined by the thread thickness and face angle. V-shaped threads have symmetric sides inclined at equal angles, generally 60°, as shown in FIG. 1. Implant screws with buttress threads have non-symmetrical sides with one face perpendicular to the axis of the screw head, or nearly so, and the other face slanted relative to the same axis at a larger angle, typically 45°. Implant screws with reverse buttress threads merely reverse the arrangement of these faces. See FIGS. 2 and 3. Square-shaped implant screws have threads with symmetrical sides perpendicular to the axis of the screw head, as shown in FIG. 4. Many practitioners prefer V-shaped threads because they provide superior surface area contact between the implant and bone, thus providing greater strength of the bond until osseointegration can take place.
FIG. 5 is a diagrammatic depiction of a conventional implant screw having V-shaped threads. The V-shaped threads are composed of root 1 and crest 3. This arrangement results in both a minor diameter 5, measured from root to root perpendicular to the screw's longitudinal axis, and a major diameter 6, measured from crest to crest perpendicular to the same axis. Pitch 7 is the distance between crest 3 and either of the two adjacent crests. The thread angle 9 is defined by the angle of opposing faces of the V-shaped threads when radiating from the root 1 to the crest 3.
Implant screws may have either a straight or tapered body. With straight body implant screws, the central body maintains a consistent, or nearly consistent, diameter throughout the length of the screw, from the screw head (or coronal neck) to the apical end of the screw. With tapered implant screws, the body diameter decreases from the coronal neck to the apical end of the screw to form a conical screw body. The tapered screw body, which is slightly larger in outer diameter than that of the pre-drilled uniform bore hole, grips the bone in a manner that compresses the bone to enhance stability. This tapered design may also be advantageous when the implant screw is placed in close proximity to the root of an adjacent tooth.
Thread depth is defined as the distance from the crest 3 of the thread to the root 1 of the thread. See FIG. 5. Many practitioners prefer to use implant screws with variable depths. Variable thread depths are advantageous because shallower thread depths are believed to allow an easier implant procedure, while deeper thread depths are believed to increase the functional surface area at the bone-implant interface, thus strengthening the connection between the implant and bone until osseointegration can occur. Therefore, progressive threads have been developed that increase in depth from the coronal neck of the implant screw to its apical end. Tapered implant screws with progressive threads, as shown in FIG. 6, often have threading with a major diameter 6 that is constant, or nearly constant, from one end of the screw to the other, while the minor diameter 5 decreases from the coronal neck of the implant screw to the apical end. Stated differently, in tapered implant screws with progressive threads, the angle of the taper may coincide with the change in thread depth, although such a correlation is not required. Alternatively, both the major and minor diameters 6, 5 may taper from the coronal neck to the apical end, with the taper of the minor diameter 5 being at a greater angle than that of the major diameter 6.
Progressive threading has been attained with square-shaped, buttress, and reverse-buttress thread types. However, manufacturing issues have, until the present invention, limited the use of progressive threading with V-shaped threads, which are popular among practitioners. In the manufacture of the screw, the process of which is schematically illustrated in FIGS. 7 and 8, as the thread cutting surface 11, which has a triangular cross-sectional geometry complimentary to the desired thread angle, is pushed deeper into the screw body 12, to a depth ranging from D1 to D2, the cutting surface 11 encroaches into the V-thread, as best shown in FIG. 8. Thus, when maintaining a constant thread pitch P1, the distance between two adjacent threads, at sufficiently high depth, the crest of the thread would be cut away, destroying the desired outer diameter of the screw. Therefore, the use of V-shaped threads on a progressive implant platform has generally not been utilized.
Therefore, there is a need for a dental implant with a V-shaped thread type that addresses the present challenges and characteristics discussed above in regard to progressive threads.